# SWD & UART (Simplified) Expansion Card **URL:** \ **Category:** Expansion Card\ **Created:** [December 30, 2022, 1:01am UTC](https://community.frame.work/t/swd-uart-simplified-expansion-card/25911 "2022-12-30T01:01:13Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Kris\_Keillor](https://avatars.discourse-cdn.com/v4/letter/k/f9ae1b/32.png) [@Kris\_Keillor](https://community.frame.work/u/Kris_Keillor)\ **Post date:** [December 30, 2022, 1:01am UTC](https://community.frame.work/t/swd-uart-simplified-expansion-card/25911/1 "2022-12-30T01:01:13Z")
I designed a fairly simple expansion card as my first. It utilizes an RP-2040 and exposes a [Serial Wire Debug](https://datasheets.raspberrypi.com/debug/debug-connector-specification.pdf) port as well as a simplified UART port, without the RTS, CTS or power pins. I rarely see the xTS pins used these days, and as for power, well, that’s something for the driven device to figure out. **[GitHub Link](https://github.com/kriskeillor/fw_rp-2040_probe)** (Docs folder contains schematic and PCB detail) ![PicoProbeCard-v1_0_0](https://us1.discourse-cdn.com/flex001/uploads/framework3/original/2X/c/c31f39af37887bd1c55aa15535993e5929dc3ad2.png) ## Features - SWD JST header with SWCLK, GND, and SWDIO pins - UART JST header with with only the TX, RX, and GND pins - Level shifters on the UART IO to accept a variety of voltages - Neopixel LED indicators for both ports - Inbound SWD exposed as test pads - Three debug GPIO pins exposed as test pads ## Outstanding issues - Level shifting… doesn’t actually work - Just now, while reading the [datasheet](https://www.mouser.com/datasheet/2/308/MC74VHC1G126_D-1387657.pdf) and [logic level docs](http://www.interfacebus.com/voltage_threshold.html), I am realizing that my naive solution won’t work. I just wired the TX and RX IO to the level-shifters like, yeah, this’ll do. What I think needs to happen is another DO switches the power supply to the level-shifters so that it can be set to 5V0 or 3V3. _Then_ it should work, but I’m going to spice it before ordering boards. Additionally, I would like to enable 1V1 and 1V8 IO, just because, but we’ll see. - Anyone have a better idea for how to approach this? I am really trying to avoid hardware switches; the face is taken up by the JST pins and Neopixels, and I don’t want the user to have to remove the card to make the change. - Limited buffer capacitors on RP2040 inputs - Due to the small card size, I removed one of the 3V3 buffer caps and two 1V1 caps. Could I have squeezed them in? Yes, but by the time I realized the space constraints, it would have required a pretty major rework. - Plan here is to build one of my other RP-2040 based board without the roughly-analogous caps and see how well they work. ## Q&A Q. Why are you making another UART board when [Josh already did that](http://community.frame.work/t/uart-expansion-card/19643)? A. Well, first of all, there’s nothing wrong with me getting some practice in with a straightforward design. But the main point of this board is the SWD port to make it friendly for RP-2040 developers, so they can flash code directly from their PC without lugging around a bulky Pico Probe. The UART port is just a bonus, since there is the space for it. I think it could be very useful to RP-2040 developers to have both in one compact package. Q: Why not use a pass-through UART chip? A: This is a great question and one area where Josh’s board will definitely outshine mine. If you want to communicate with electronics _directly_ from your computer, this may not be the best option. (It may be possible with a pass-through executed in software? Not sure.) The UART port will likely be used more as a unit tester or logger than a full communication port. Q. Why NeoPixels? A. Because NeoPixels are cool and I’ve wanted to incorporate them into a project for a while. Considering each board will likely be hand-made by me, the true cost is in labor, not materials. So it doesn’t make a huge difference to spend 70c instead of 7c on the indicator lights. NeoPixel will give greater flexibility; instead of a single color communicating information from the blinking frequency, a variety of status can be indicated from diverse color patterns. The statuses will relate to the SWD and UART bridges as well as status of the card. * * * That’s what I have for now. Not ready to order boards due to the level shifting issue. But while I am tackling that, I’d like to run a poll and determine which of the above features folks actually find useful. Please answer if you could envision an RP-2040 based SWD port expansion card as a useful part of Frame.Work toolkit! **What features would you like to see in an RP-2040 SWDebugger Dev Card?** _Poll ([view on site](https://community.frame.work/t/swd-uart-simplified-expansion-card/25911/1))_ Cheers, Kris K ---
**Author:** ![nrp](https://sea1.discourse-cdn.com/flex001/user_avatar/community.frame.work/nrp/32/352_2.png) [@nrp](https://community.frame.work/u/nrp)\ **Post date:** [February 3, 2023, 4:45pm UTC](https://community.frame.work/t/swd-uart-simplified-expansion-card/25911/2 "2023-02-03T16:45:26Z")
Awesome to see this! The RP2040 is a great MCU for Expansion Card use.